how to understand the relation of these transistors?

[xuexucheng]

how to understand the relation of these transistors?

What I can not understand is how to determine the voltage of AAA node and how to determine the current of Q4 Q2 Q1 ?

Thanks in advance.

 

[leo_o2]

AAA voltage is -2Vbe.

 

[skowe26]

Hi, you can start by calculating the current through R13.

 

[xuexucheng]

Hi, you can start by calculating the current through R13.

Yes, it is easy to calculate the current through R13.
But how to calculate the voltage of AAA and current through the Q4 Q2 Q1 ?
How about R1 affect the bias voltage and current??

 

[erikl]

Yes, it is easy to calculate the current through R13.
But how to calculate the voltage of AAA and current through the Q4 Q2 Q1 ?
How about R1 affect the bias voltage and current??

As Leo told you above, the AAA voltage is -2Vbe or about -1.3 .. -1.4V .

You don't need to calculate the current through R13 in order to get the current through Q4 Q2 Q1 :
The collector voltage of Q1 = -Vbe(Q2) - Vcesat(Q1) ≈ -0.7V - 0.3V = -1V
Hence the current through Q4 Q2 Q1 & R1 is 14mA .

 

[FvM]

The collector voltage of Q1 = -Vbe(Q2) - Vcesat(Q1) ≈ -0.7V - 0.3V = -1V
Hence the current through Q4 Q2 Q1 & R1 is 14mA .

No. All transistors are carrying the same current (ignoring small differences caused by finite B). It's about 0.73 mA - and set by R13.

 

[geody]

Fvm is right. Q1 Q2 Q4 & Q9 actually make a current mirror. So all components carry the same current.
Here R1 should be the load resistor. it can not affect the current or other voltage.

 

[xuexucheng]

No. All transistors are carrying the same current (ignoring small differences caused by finite B). It's about 0.73 mA - and set by R13.

1. How to caculate the voltage?

2. What confused me is that why the current of Q4 is determined by Q9? Why the current of Q9 is not determined by Q4?

3. In textbook the current is determined(mirorred) by the diode connected transistor.

 

[FvM]

What confused me is that why the current of Q4 is determined by Q9?

It isn't. Instead the current of Q4 is controlled by Q1. It's not a simple current mirror, it's a kind of Wilson current mirror (with additional cascode). Review your text books for it.

Wilson current mirror - Wikipedia, the free encyclopedia

 

[xuexucheng]

It isn't. Instead the current of Q4 is controlled by Q1. It's not a simple current mirror, it's a kind of Wilson current mirror (with additional cascode). Review your text books for it.

Wilson current mirror - Wikipedia, the free encyclopedia

I have learn the Wilson current mirror . but my circuit is not same with the Wilson current mirror . still confused.

 

[FvM]

but my circuit is not same with the Wilson current mirror

Yes, I mentioned it has an additional cascode. If you understand the Wilson circuit, you'll see, that the control method of your circuit is basically the same.

 

[totowo]

I don't think it's the same.
The Wilson don't need any cascode because there MUST be all Vbe drop. A cascode here only shows a drop of Vce, it's doesn't make sense.

Yes, I mentioned it has an additional cascode. If you understand the Wilson circuit, you'll see, that the control method of your circuit is basically the same.

 

[FvM]

I don't think it's the same.
The Wilson don't need any cascode because there MUST be all Vbe drop. A cascode here only shows a drop of Vce, it's doesn't make sense.

I don't feel a need to argue how much different the circuit is from a Wilson current source. Obviously a Wilson current mirror doesn't need a cascode. My point was, that the current control mechanism is similar to a Wilson current source, it's not changed by an additional cascode transistor. Did you understand this point?

However I don't know the purpose of this circuit, I was just answering the questions about currents and why is it so.